The underlying hypothesis to be tested in the present proposal is that there is a specific set of irradiation conditions for laser light that most efficiently and effectively interacts with dental hard tissues. The efficient conversion of light energy to heat when a laser beam interacts with dental hard tissues is strongly wavelength dependent. The potential is excellent for the clinical use of specific lasers to provide protection against caries and for the treatment of early lesions. Before the clinical application can be realized the scientific basis for the choice for the specific conditions must be established and safety issues must be addressed. The purpose of the studies proposed here is to establish that basis. Lasers used in the studies have been selected to examine the most likely useful wavelength from IR though UV, namely CO2(9.3-11.0 micro m), Nd:YLF (1.o5 micro m, comparable to Nd:YAG), and its harmonics at 0.53 and 0.35 micro m. Diffuse transmission ad backscattering will be examined at all wavelengths using enamel, dentin and synthetic carbonated apatite (CAP). Scattering and absorption coefficients will be determined. We will determine quantitatively the heat propagation into enamel and dentin of various thicknesses using selected laser irradiation conditions (pulse length, energy, rep-rate, number of pulses). Laser interactions will be examined (using conditions determined from the optical and thermal studies) with (1) surfaces of tooth enamel, tooth roots and synthetic carbonated apatite, and (2) caries-like lesions in (a) dental enamel smooth and occlusal surfaces, (b) root surfaces with and without cementum, all with or without fluoride treatment. In all cases the experiments with caries-like lesions will be followed by studies to measure the inhibition of lesion progression using a Ph cycle model. Results will be assessed using a combination of polarized light microscopy, quantitative microradiography, x-ray diffraction, infrared spectroscopy, scanning electron microscopy, and chemical analysis. An indication of safety of the treatments will be assessed from the thermal studies and by temperature measurements in the pulp chamber and at the outer surfaces. All data will be analyzed statistically and interpreted to indicate whether specific irradiation conditions are associated with specific outcomes in dental hard tissues and mineral. It is anticipated that the present study will provide a range of laser irradiation conditions that will optimize the beneficial effects of increasing caries resistance with the ultimate aim of developing a laser system(s) for use as an improved method for the prevention and treatment of pit and fissure caries, early enamel caries, and root caries.